The invention relates to a process for continuously performing polymerization processes, especially for the homo- or copolymerization of thermoplastics and elastomers, and a system therefore.
A considerable portion of polymerization reactions, especially for the preparation of homo- and copolymeric thermoplastics and elastomers, are performed commercially as a slurry or solution process in one or more series-connected, continuous, backmixed, vertical stirred tank reactors, known as “CSTRs”, continuous stirred tank reactors.
These stirred tank reactors have the task of very homogeneously distributing the monomers, catalysts and initiators in a solvent/diluent under precisely defined process conditions, such as temperature and pressure, in order that the reaction proceeds in a controlled manner, a uniform product quality with the desired molar mass is formed and the heat of reaction is also controlled.
The problem of these stirred tank reactors consists in the fact that it is only possible to process products with a low apparent viscosity. With rising concentration of the polymer in the solvent/diluent, the apparent viscosity of the reaction mixture rises such that the stirrer finally cannot reaction mixture rises such that the stirrer finally cannot achieve sufficient convective flow. The consequence thereof is inhomogeneous distribution of the monomers. This leads to lump formation, poor molar mass distribution, caking, local overheating, up to and including an uncontrolled reaction of the entire reactor contents.
A further problem of stirred tank reactors is foam formation in the case of individual products, which can lead to blockages in the vapor draws.
The above-mentioned process risks lead to the fact that stirred tank reactors are operated with a large excess of solvents/diluents up to approx. 90% of the reaction mixture, or only conversions of less than 50% can be achieved in bulk polymerizations. As a consequence thereof, additional process steps for the mechanical/thermal removal of the diluent/solvent/monomer or for the post-reaction are necessary. This is generally effected in dewatering screws, condensation and drying systems and maturing tanks. They mean high capital, energy and operating costs. There are even new polymers which are not processable with a water stripping process!
Bulk polymerizations are also performed continuously in single-shaft or multishaft extruders (for example from Werner Pfleiderer, Buss-Kneter, Welding Engineers, etc.). These apparatus are designed for polymerizations in the viscous phase up to high conversions. They are constructed as continuous plug flow reactors and accordingly have a large L/D ratio of from >5 to approx. 40
The following problems occur here:
a) In the case of slow polymer reactions with reaction times of >5 minutes, in which the reaction mixture remains in the liquid state for a long period, the plug flow cannot be maintained. The very different rheological properties between the monomers and polymers prevent uniform product transport, which leads to undesired quality variations.
b) The high exothermicity of many polymerization processes and the dissipated kneading energy frequently make it necessary to remove these energies by means of evaporative cooling. This is done by evaporating a portion of the monomer or of an added solvent/diluent, condensing it in an external condenser and recycling the condensate into the reactor. Owing to the large L/D ratio and the large screw cross section for construction reasons, only very limited free cross-sectional areas are available for the removal of vapors. This leads to the undesired entrainment of polymers into the vapor lines and into the reflux condenser and, as a consequence thereof, to blockages.
c) In the preparation of (co)polymers from a plurality of different monomers, an additional complicating factor is that mainly the monomer with the lowest boiling point evaporates for the evaporative cooling, so that a shift in the monomer concentrations is established in the reactor, especially in the region of the entry orifice of the condensate reflux. This is generally undesired.
d) Another disadvantage is that the free product volume of screws is limited to about 1.5 m3 for mechanical construction reasons, so that only low throughputs can be achieved in reactions with residence times of >5 minutes, which requires the installation of a plurality of parallel lines with correspondingly higher capital and operating costs.
A further means of performing bulk polymerizations up to high conversions is described in U.S. Pat. No. 5,372,418. Here, co- or contrarotating multiscrew extruders with non-meshing screws or screw pairs which convey in opposite directions for the polymerization of the monomers by backmixing with the polymer in the viscous phase are described. These apparatuses are capable in principle of performing polymerization processes up to high conversions and simultaneously of avoiding the above-described disadvantages a) (collapse of the plug flow) and c) (formulation shift through reflux) from the plug flow extruder. However, the above-described problems b) (reduced free cross section) and d) (size) still remain unsolved.